This invention relates to piston sealing rings for reciprocating fluid pumps or compressors or vacuum pumps and, in particular, to a piston sealing ring assembly of especial, but not exclusive, application to single or double acting oil-free reciprocating air and/or gas compressors.
The basic operational principle of fluid reciprocating pumps or compressors is to draw fluid into a cylinder through a one-way valve by linear movement of a piston sliding in sealed relationship within the cylinder and then to expel the fluid from the cylinder through a further one-way valve by reversing the direction of movement of the piston within the cylinder. The efficiency of the pump is determined to some extent by the quality of the seal between the piston and cylinder as any fluid that is forced between them and past the piston from the higher pressure side to the lower pressure side during the pumping or compression stroke uses an energy input that is then no longer available to usefully pump or compress the fluid.
One approach to sealing the piston/cylinder interface is to size the piston to be a close fit in the cylinder and lubricate the small gap with an oil film using oil from a sump which enters the cylinder on the side of the piston away from the fluid being pumped or compressed. Generally an oil scraper is provided to recirculate oil spread up the wall of the cylinder back to the sump via the interior of the piston.
The gap between the piston and cylinder is sealed against ingress of the fluid being pumped or compressed by one or more piston sealing rings each located in a respective annular retaining recess.
Known piston sealing rings for use in such oil lubricated pumps or compressors are so called xe2x80x9ccutxe2x80x9d sealing rings, for example of cast iron, the cut being made to remove a small section of the ring to form a circumferential ring gap in the ring and having an outer diameter a little larger than the inner diameter of the cylinder and as shown in FIG. 1. The ring gap allows the ring""s diameter to be reduced for insertion into the cylinder while being retained in the annular recess whereafter it is free to expand outwards by virtue of its inherent resilience towards its original diameter and into sealing contact with the cylinder. Such a piston sealing ring is shown at FIG. 1.
As the piston sealing ring undergoes thermal expansion due to heating when in use, the gap in the ring reduces in size as the ring is constrained from further radial expansion by the cylinder. The circumferential extent of the gap is therefore chosen so that it will not close up when the pump or compressor is operated within its design parameters. The necessary presence of the ring gap provides a small leakage path for the pressurised fluid past the piston sealing ring as follows.
The fluid under compression relative to the fluid at the other side of the piston acts to force the piston sealing ring into sealing contact with the annular wall at the lower pressure side of the recess. It will not be in sealing contact with the annular wall at the higher pressure side of the recess nor with the inner circumferential surface of the recess (due to the outward expansion of the ring). Assuming, for descriptive convenience only, that the higher pressure side of the piston is the upper side there is a leakage path for the fluid down the gap between the upper portion of the piston and cylinder, between the piston sealing ring and the upper annular surface of recess, behind the piston sealing ring between its inner circumferential surface and the recess, though the ring gap in the piston sealing ring, and finally down the gap between the piston and cylinder gap below the piston sealing ring.
The leakage rate due to this leakage path is primarily determined by the area of overlap of the ring gap at the sealed (lower) side of the piston sealing ring and the gap between the piston and cylinder. For oil-sealed fluid pumps or compressors the area of overlap is sufficiently small, due to the close fit of the piston in the cylinder, that the leakages produced in most applications can be considered to be small.
In some applications it is necessary to ensure there is no contamination by lubricating oil of the fluid being pumped or compressed in which case an oil-free piston/cylinder arrangement can be employed. Known arrangements of oil-free pumps or compressors or vacuum pumps have a piston with an outer diameter some 1 to 3 mm smaller than the cylinder and which are provided with a rider ring retained in an annular recess in the periphery of the piston which is a closer but non-sealing fit in the cylinder to centre the piston in the cylinder as it moves within it. In such oil-free piston/cylinder arrangements it is usual to seal the gap between the piston and cylinder by a cut piston sealing ring, in the manner described above in relation to oil-lubricated piston/cylinder arrangements, but the piston sealing ring has to be of a material which is self-lubricating relative to the material of the cylinder, for example carbon graphite filled PTFE.
The leakage path discussed above in relation to an oil-lubricated apparatus with a cut sealing ring also exists with oil-free piston/cylinder apparatus but the leakage is greater, all other things being equal, because the area of overlap of the ring gap in the piston sealing ring (because of the increase in thermal expansion rate of PTFE type rings) and gap between the piston/cylinder is greater (because of the increased size of the latter) and can be sufficiently large to have a significant effect on the efficiency of such fluid pumps or compressors.
It is also known to form a piston ring assembly for an oil-free fluid pump or compressor or vacuum pump by stacking two piston sealing rings each with a ring gap as described above but circumferentially offset from each other as shown in FIG. 2. However, the leakage is still determined by the area of the overlap of the ring gap at the low pressure side of the sealing piston ring and the gap between the piston/cylinder as the higher pressure fluid can still enter the interior volume between the inner circumferential surface of the piston sealing ring and the annular recess and then pass to the gap at the lower pressure side to the leak between the piston/cylinder as before.
A known prior art piston sealing ring which can provide better sealing has a finger-like extension which extends from one side of the ring gap into sliding engagement with a complimentary notch at the other side of the ring gap as shown in FIG. 3. In this case the overlap region of the ring gap and the piston/cylinder gap at the lower pressure side of the piston sealing ring (which is to the upper side of the piston sealing ring of FIG. 3 in use) can, if the sealing ring is accurately manufactured, communicate with the volume between the piston sealing ring and annular recess only by passing between the finger and the surfaces at the notch against which it sealed. The area of overlap of the ring gap and the gap between the piston and cylinder therefore does not determine the leakage rate of this piston sealing ring rather it is determined by the quality of seal provided by the finger and notch.
There are, however, a number of disadvantages associated with this last described piston sealing ring.
It is necessary to employ two-axis milling operations to create the interlocking finger/notch structure.
The piston sealing ring notch and finger must be accurately dimensioned to avoid unwanted leakage. If the notch is too shallow in the axial direction the axial seal against the lower pressure side of the piston sealing ring recess will be broken. If the notch is too deep in the radial direction the circumferential seal between the finger and notch could be broken when the ring expands to conform to a cylinder""s inner diameter. If the notch is too shallow in the radial direction the circumferential seal of the sealing ring to the cylinder will be adversely affected, although this would improve as the finger wears differentially until it become sized to the radial depth of the notch.
Further, engineering constraints require a given thickness of material for the finger (dependent on operating conditions and ring diameter and material, for example) and the axial depth of such piston sealing rings generally has to be about twice the finger depth.
It is also to be noted that the finger is formed as part of the outer circumferential portion of the sealing ring. This means the finger is exposed to wear in use and needs to be radially wide enough initially to be of sufficient mechanical strength when worn during use to provide a satisfactory useful life to the ring.
These disadvantages are present whether the piston sealing ring is used to seal a single or double action fluid pump or compressor, but in the latter case there are additional disadvantages of the above described piston sealing ring with a finger/notch sealing arrangement. As this sealing ring is only able to seal in one direction, it is necessary to use two of them, each located in a corresponding annular recess in the piston to seal a double action pump or compressor. This requires a total axial depth of sealing ring in the region of four times the axial depth of the finger and two distinct annular recesses to hold them, all of which place dimensional constraints on the minimum length of the piston.
The present invention seeks to provide a piston sealing ring assembly which addresses these disadvantages of the prior art seals, and particularly these described in relation to the piston sealing ring of FIG. 3.
Accordingly there is provided a piston sealing ring assembly comprising a first and second cut piston sealing ring each bounded by first and second planar surfaces, the first piston sealing ring having an overlapped cut with generally circumferentially extending inner and outer overlapping portions which contact each other to form a continuous seal extending from the first surface to the second surface of the first piston sealing ring and separating an inner ring gap and an outer ring gap, the periphery of the outer ring gap of the first piston sealing ring at the first surface being sealed by the second surface of the second piston sealing ring.
The piston sealing ring assembly preferably is such that the outer overlapped portion of the overlapped cut is wider in a radial direction than the inner portion of the overlapped cut, for example twice as wide.
The inner portion of the overlapped cut is preferably formed so as to be resiliently biased towards the outer portion of the overlapped cut to provide a more secure sealing arrangement.
The piston sealing ring assembly may be such that the second piston sealing ring has an overlapped cut with generally circumferentially extending inner and outer overlapping portions which contact each other to form a continuous seal extending from the first surface to the second surface of the second piston sealing ring, the periphery of the outer ring gap of the second piston sealing ring at the second surface being sealed by the first surface of the first piston sealing ring. This provides a compact seal which can seal in two directions for use in a double-action gas compressor, for example.
Preferably, an interlocking means is provided for retaining the first and second piston sealing rings in a predetermined circumferential orientation. For example, the interlocking means may comprise a pin fixed in one of the piston sealing rings and engageable in a hole in the other piston sealing ring.
The piston sealing rings may be formed from a PTFE or a PTFE compound or other suitable self-lubricating material.